A polarized xenon gas can be produced by, for example, an optical-pumping method that uses rubidium or a like alkali metal. The polarized xenon gas is suitably used for NMR (nuclear magnetic resonance) devices, MRI (magnetic resonance imaging) devices and the like. In order to obtain a highly polarized xenon gas, a diluent gas composed of nitrogen, helium or like inert gas is usually added to a xenon gas during its production process. By separating the resulting polarized xenon gas from the diluent gas and concentrating it, a highly concentrated polarized xenon gas can be obtained.
Using liquid nitrogen to solidify a polarized xenon gas has been proposed as a method for concentrating the polarized xenon gas by separation of a diluent gas. However, this method has a drawback in that the polarization rate decreases in the process of solidifying and volatilizing the xenon. Furthermore, in this method, the xenon gas has to be once solidified and removed. This makes the continuous production of polarized xenon gas impossible; therefore, its use is limited to NMR or MRI experiments conducted in a batch mode. More specifically, the increase in the polarization rate by mixing a diluent gas becomes remarkable when the concentration of xenon gas is as low as 10% or less (Non-patent Literature 1). But the separation of a large amount of diluent gas, i.e., more than 10 times the amount of the xenon, becomes necessary since a high polarization rate is obtained under a high dilution condition. But, solidifying xenon is an only effective method to realize the separation under such a condition. Therefore, measurement in NMR or MRI has been conducted only in a batch mode.
In contrast, a continuous supply of a concentrated xenon gas with a high polarization rate is necessary to conduct on-line measurement in NMR or MRI. Patent Literature 1 discloses a device that uses a selectively permeable membrane which transmits a diluent gas only and does not transmit polarized xenon gas so that the polarized xenon gas, which does not pass through the membrane, is extracted by suction. However, although the device disclosed in Patent Literature 1 succeeded in obtaining a higher concentration of polarized gas by transmitting only the diluent gas and the polarized gas is concentrated to approximately twofold in xenon concentration, the polarization rate of xenon gas is reduced to ½. Therefore, a reduction in the polarization rate is serious and the total sensitivity (polarization rate×concentration) is not improved as a whole.
Patent Literature 2 discloses that highly concentrated xenon gas can be obtained without the solidification of polarized xenon by using a high concentration xenon gas such as 80 to 100% in xenon (the remaining portion is nitrogen gas) as a source gas to be hyperpolarized; however, the polarization rate is as low as 1 to 2% (estimated from the data disclosed in Non-patent Literature 2).